Image display devices, such as flat panel display devices, are subject to contrast ratio reduction and glare due to reflections of ambient light at transparent face plates and underlying cathodoluminescent coatings. Various structures and treatments have been used to address this problem, including the provision of surface irregularities and patterns, to function as ambient light scattering elements that redirect reflections of incident ambient light away from the angle of view of the viewer. Examples of such treatments are given in U.S. Pat. Nos. 4,972,117 and 5,240,748. For liquid crystal displays (LCDs), available viewer viewing angles tend to be limited, so scattering of glare causing reflections out of the field of view has some use; though, the trend is to increase available viewer angles. Moreover, though scattering reduces reflection concentrations at any given angle of reflection, non-productive light (i.e., light that is not part of the image-formative process) is still returned to the viewer.
U.S. Pat. No. 5,206,746 discloses a transparent optical device comprising a side-by-side array of triangular prisms that is interposed between spaced liquid crystal and backlighting components of a liquid crystal display. The prism bases serve as apertures for admission of incident ambient light into channels bounded by converging prism side surfaces. The prism apexes (called "valley bottom portions" in the '746 patent) are covered with light absorbing material. Ambient light incident on the bases of the prisms is multiply reflected toward the apexes and absorbed by the absorbing material. On the other hand, light traveling in the opposite direction from the backlighting source and incident on the apexes is relatively unaffected and enabled to pass through to the viewer, or be scattered, in accordance with the transparent or scattering mode imparted to the liquid crystals. The full disclosure of the '746 patent is incorporated herein by reference.
The '746 prisms are formed by machining, casting, pressing, injection molding or similar processes for which sharp peaks are not obtained. A trade-off is, therefore, required between sizing and covering truncations or "cuts" with material for maximum ambient light absorption, and minimizing obstruction to transmission of image-forming backlighting in the other direction. Moreover, the size and pitch of the '746 prisms is on the order of millimeters; thus, careful positioning is required to avoid blocking pixel rows/columns or introducing moire interference patterns ( see, e.g., discussion in the '117 patent).
Flat panel displays are widely used as image display screens for laptop and notebook computers. In this context, the term "flat" used herein is a reference to thinness (viz. compared to traditional electron gun cathode ray tube displays), not planarity. That term is therefore intended to encompass thin non-planar, curved displays, as well as thin planar displays. Flat panel displays of the so-called "field emission display" (FED) type, such as described in U.S. Pat. Nos. 4,857,799, 5,103,144 and 5,225,820, have transparent face plates including anode electrodes and cathodoluminescent coatings. Such displays include a matrix array of individually addressable light generating means. An emitter plate, spaced from the face plate, has a plurality of conductive stripes, each with a multiplicity of spaced-apart electron emitting tips which serve as cathodes and project upwardly toward the face plate. An electrically conductive extraction (i.e. gate) electrode arrangement is positioned on the emitter plate adjacent the tips to generate and control the electron emission. The extraction electrode arrangement comprises a large number of individually addressable, cross-stripes which are orthogonal to the cathode stripes and which include apertures through which emitted electrons may pass.
Because it is desired to be able to operate them at low power and under bright outdoor light, FED displays are especially sensitive to the problem of ambient light reflection. The cathodoluminescent coating used most often on FED displays is a layer of granular phosphor. While only as little as 3% of incident ambient light may reflect back from the glass-air boundaries of the plate, as much as 50% may be reflected by the phosphor layer. This severely restricts the contrast ratio available even in an undarkened room. In fact, under normal outdoor or room lighting conditions, the unlit ("off" condition) conventional FED screen appears white, rather than gray or black.
The anode of a conventional FED display comprises a thin film of electrically conductive material which covers the interior surface of the face plate. For a monochrome display, the anode film usually takes the form of a continuous layer across the surface of the face plate. For a color display, as in U.S. Pat. No. 5,225,820, the anode is segmented into three electrically isolated combs. Each comb comprises a plurality of connected bands or stripes covered with phosphor particles which luminesce in a different respective one of the three primary colors--red, blue and green. Because of the reflective nature of metal, and in order to be able to view the luminescing phosphor through the anode, conventional FED designs require that the anode be formed of a transparent conductive material, such as indium-tin-oxide (ITO). Such transparent material is, however, less conductive than aluminum and other traditional non-transparent conductive materials.
Arcing between different color phosphor anode stripes is minimized in FED displays by drawing and maintaining a vacuum in the space between anode and emitter plates. However, voltage standoff between different color combs at high voltages is still a problem because of surface leakage between coplanar razor edges of the separate electrode depositions disposed across the smooth back surface of the shared face plate. Such leakage is a precursor to arcing. There is, thus, also a need to minimize leakage between adjacent stripes of different color combs.